1. Field of the Invention
This invention relates to a method and apparatus for performing inter-system handover between telecommunications systems, and especially to the handling of classmarks or like indicators.
In a mobile radio telecommunications system user's terminals communicate by radio with base stations which are connected to control units which control the operation of the mobile radio system and to the wider telecommunications network. The user terminal is often referred to as a mobile station (MS) which could be a mobile phone; however, the location of the MS could be fixed.
FIG. 1 shows schematically the configuration of a typical cellular radio telecommunications network. The network comprises a number of base-stations (BSs) 1, 2, 3 etc. Each base-station has a radio transceiver capable of transmitting radio signals to and receiving radio signals from the area of a cell 4, 5 etc. next to the base-station. By means of these signals the base-station can communicate with a mobile station (MS) 6 in that cell, which itself includes a radio transceiver. Each base station is connected via a base station controller (BSC) 9 to a mobile switching centre (MSC) 7, which is linked in turn to the public telephone network 8. By means of this system a user of the MS 6 can establish a telephone call to the public network 8 via the BS in whose cell the MS is located.
The location of the MS could be fixed (for example if it is providing radio communications for a fixed building) or the MS could be moveable (for example if it is a hand portable transceiver or “mobile phone”). When the MS is moveable it may move between cells of the cellular radio system. As it moves from one cell (the “old cell”) to another cell (the “new cell”) there is a need to hand it over from communication with the BS of the old cell to the BS of the new cell without dropping the call due to a break in communications between the mobile station and the network. This process is known as handover. A need can also arise to hand over a MS whose location is fixed, for example if atmospheric conditions affect its communications with the old BS and call quality can be improved by handing it over to another BS or if there is a need to free up capacity of the old BS.
In a conventional cellular radio system handover is controlled by the BSC. Handover can be initiated by the network dependent, for example, on the quality of the signalling between the MS and the old and new BSs.
When a mobile station first established connection with the radio system, which could happen when it is turned or moves into an area of radio coverage of one of the base stations, the mobile station may send a message to the radio system to report its capabilities so as to allow the system to properly accommodate the mobile station.
In the GSM (Global System for Mobile Telecommunications) telecommunications system the reporting of such capabilities is done by way of messages indicating classmark information. In the GSM system the classmark information is divided into three separate classmark parameters, each of which includes information related both to the radio system and to the core network. One of these parameters is the MS classmark 2 parameter. That parameter is sent to the network in the paging response message (which a mobile station sends in response to a paging request message from the GSM network) or in the CM (connection management) service request message. Another of the parameters is the MS classmark 3 parameter. That parameter is sent in response to an enquiry from the mobile switching centre (MSC) of the GSM network (which is generally only made when the information is needed, for example to find the mobile station's encryption capabilities), or spontaneously if the MS supports services indicated in classmark 3.
In the proposed UMTS (Universal Mobile Telecommunications System) telecommunications system it is proposed that the classmark information will be rearranged into only two parameters: an access network classmark parameter (known as the AN classmark) and a core network classmark parameter (known as the CN classmark). This has been proposed because in the proposed UMTS system there is to be a clearer distinction between the radio access network and the core network than there is in the GSM system, and because the UMTS system is not to be subject to the restrictions on message size for classmark parameters that are enforced in the GSM system. An additional difference is that certain information in the GSM classmark that is specific to the GSM system is not required in the UMTS classmark. Similarly, the UMTS classmark includes additional information that is not present in the GSM classmark.
When a new cellular network is being introduced it can take some time to install all the base-stations and associated apparatus. Therefore, there is a delay before the new network provides full geographical coverage. FIG. 2 illustrates the situation: an existing cellular network provides full geographical coverage by means of cells 20-27 but the new cellular network provides incomplete geographical coverage by means of only cells 28 and 29. This presents a significant commercial problem for the operator of the new network. If the new network is launched for use before its geographical coverage is complete then customers will be dissatisfied by its inferior coverage to the old network. However, the cost of the infrastructure of the new network is high and no return can be gained on it until it is in use.
It has been proposed to tackle this problem by allowing mobile stations using the new network to be handed over to cells of the old network when they move outside the coverage of the new network. For instance, when a mobile station moves from 30 to 31 in FIG. 2 it could be handed over from the base station of cell 28 (in the new network) to that of cell 21 (in the old network). This is known as inter-system handover.
To allow the UMTS system to be introduced smoothly and without significant disruption to current users of the GSM system it is proposed that the new UMTS system will be capable of a substantial level of interworking with the existing GSM system. One aspect of this is the aim to support inter-system handover of a mobile station between the GSM and UMTS systems. However, the differences in the treatment of classmarks between the GSM and UMTS systems presents a significant barrier to inter-system handover. In order to allow for inter-system handover between the UMTS system and the GSM system there is a need to address the differences between the GSM and UMTS classmark arrangements. One way that has been proposed to address these differences is for the mobile station to provide all its classmark information (for both GSM and UMTS) to the network in all circumstances. However, this would increase signalling load unnecessarily in cases when no inter-system handover is subsequently performed.
There is therefore a need for an improved method of dealing with data such as classmark information for facilitating inter-system handovers.